The present invention is directed to bicycles and, more particularly, to a method and apparatus for controlling an automatic bicycle transmission.
A typical bicycle transmission may include a plurality of front sprockets that rotate with the pedal cranks, a plurality of rear sprockets that rotate with the rear wheel, and a chain that engages one of the front sprockets and one of the rear sprockets. A front derailleur may be mounted to the bicycle frame for shifting the chain among the plurality of front sprockets, and a rear derailleur may be mounted to the bicycle frame for shifting the chain among the plurality of rear sprockets. Manually operated switches or levers may control the front and rear derailleurs. Some modem bicycles use small electric motors to control the bicycle transmission. The motors may be controlled manually by the foregoing switches or levers, or automatically based on bicycle speed and/or cadence.
A number of automatically controlled bicycle transmissions have been designed. For example, U.S. Pat. No. 4,490,127 issued to Matsumoto discloses one type of automatically controlled bicycle transmission. In that system, a wheel speed sensor (comprising a magnet mounted to the wheel and a magnetic sensor mounted to the frame) is used to determine wheel speed (RPM), and a pedal crank rotation sensor (comprising a magnet mounted to the pedal cranks and a magnetic sensor mounted to the frame) is used to determine pedal crank cadence (RPM). A rear derailleur is operated to shift gears based either on bicycle speed or pedal crank cadence. When the derailleur is operated to shift gears based on bicycle speed, upper and lower wheel speed limits are calculated using the equations V(upper)=aX and V(lower)=a(X−1)−b. In these equations, X is the number of the sprocket engaged, and a and b are values determined empirically and set by switches on the computer console. When the derailleur is operated to shift gears based on cadence, single upper and lower cadence limits are set for all gears.
U.S. Pat. No. 5,261,858 issued to Browning discloses an automatically controlled bicycle transmission for controlling the operation of both front and rear derailleurs. In this system, a table is prepared listing the gear ratios for various combinations of front and rear derailleur positions, and a wheel speed that produces a desired cadence is set for each gear ratio. The table is sorted from lowest to highest gear ratio, and the front and rear derailleurs are operated to move from lower to higher gear ratios (or vice versa) to maintain a desired cadence. The shift points are set to halfway between the desired wheel speeds for adjacent gear ratios.
U.S. Pat. No. 5,728,017 issued to Bellio, et al also discloses an automatically controlled bicycle transmission for controlling the operation of both front and rear derailleurs. In this system, a wheel speed sensor is used to determine wheel speed, and a pedal crank rotation sensor is used to determine pedal crank cadence. The rider enters a calibration mode and rides the bicycle in each front and rear derailleur combination. The system then calculates a gear ratio for each combination of front and rear derailleur positions using the formula Gear Ratio=Wheel RPM/Cadence. The resulting gear ratios are sorted from numerically lowest to numerically highest and stored together with their corresponding front and rear derailleur positions in a memory. The rider also specifies low and high cadence limits. During automatic operation of the bicycle, the system calculates a theoretical cadence from the wheel speed and current gear ratio. The front and rear derailleurs are operated to upshift when the theoretical cadence is too low, and the front and rear derailleurs are operated to downshift when the theoretical cadence is too high.
In U.S. Pat. No. 4,490,127, the wheel speed at which derailleur shifting occurs is calculated in a somewhat complicated and arbitrary manner, and the calculations do not seem to take into account the actual sprocket sizes used on the bicycle. In both U.S. Pat. No. 5,261,858 and 5,728,017, both front and rear derailleurs may have to be operated to shift a single step, and complicated calculations must be made to operate the system based on a desired cadence. This is especially true of the system disclosed in U.S. Pat. No. 5,728,017, which requires the rider to pedal the bicycle to obtain actual cadence values to set cadence limits, and then the computer calculates theoretical cadence values (using gear ratios and current wheel speeds) that are compared to the previously set cadence limits.